1. Field of the Invention
The present invention relates generally to a Frequency Division Multiple Access (FDMA)-based wireless communication system, and in particular, to a method and apparatus for transmitting and receiving Channel Sounding Reference Signals (CS RS).
2. Description of the Related Art
Recently, in mobile communication systems, intensive research has been conducted on Orthogonal Frequency Division Multiple Access (OFDMA) or Single Carrier-Frequency Division Multiple Access (SC-FDMA) as a scheme suitable for high-speed data transmission in wireless channels.
Presently, the OFDM and SC-FDMA technologies are applied in the downlink and uplink of the Evolved UMTS Terrestrial Radio Access (EUTRA) standard based on Universal Mobile Telecommunication Services (UMTS) defined by the 3rd Generation Partnership Project (3GPP).
SC-FDMA, a technology that is based on single-carrier transmission while guaranteeing orthogonality between multiple access users like OFDM, is advantageous in that a Peak-to-Average Power Ratio (PAPR) of transmission signals is very low. Therefore, SC-FDMA, when it is applied to the mobile communication system, can bring improvement of the cell coverage due to its low PAPR, compared to the OFDM technology.
FIG. 1 illustrates a structure of a general SC-FDMA transmitter and a slot structure, in which Fast Fourier Transform (FFT) 103 and Inverse Fast Fourier Transform (IFFT) 105 are used.
Referring to FIG. 1, a difference between OFDM and SC-FDMA will be considered in terms of the transmitter structure. Aside from IFFT 105 used for multi-carrier transmission in an OFDM transmitter, FFT 103 further exists in front of the IFFT 105 in an SC-FDMA transmitter. Here, M modulation symbols 100 constitute one block, and the block is input to the FFT 103 with a size M. Each of the blocks will be referred to herein as a ‘Long Block (LB)’, and 7 LBs constitute one 0.5-ms slot 102.
Signals output from the FFT 103 are applied to the IFFT 105 as inputs having consecutive indexes (See 104), where the signals undergo inverse Fourier transform, and then are converted into an analog signal (See 106) before being transmitted. An input/output size N of the IFFT 105 is greater than an input/output size M of the FFT 103. The SC-FDMA transmission signal has a lower PAPR than the OFDM signal because the signal processed by means of the FFT 103 and IFFT 105 has single-carrier characteristics.
FIG. 2 illustrates exemplary resource partitioning in the frequency-time domain in a EUTRA SC-FDMA system.
Referring to FIG. 2, a system bandwidth 201 is 10 MHz, and a total of 50 Resource Units (RUs) 202 exist in the system bandwidth 201. Each RU 202 is composed of 12 subcarriers 203, can have 14 LBs 204, and is a basic scheduling unit for data transmission. The 14 LBs 204 constitute one 1-ms subframe 205.
FIG. 3 is a diagram illustrating resource allocation for transmission of a control channel and a data channel in the EUTRA uplink based on the resource partitioning structure of FIG. 2.
Referring to FIG. 3, control information, such as ACKnowledge (ACK)/Negative ACK (NACK) representative of response signals for a Hybrid Automatic Repeat reQuest (HARQ) operation for downlink data and Channel Quality Indication (CQI) representative of channel state information for downlink data scheduling, is transmitted through the RUs located in both ends, i.e., RU#1 and RU#50 of the system band. Meanwhile, information such as data, Random Access CHannel (RACH) and other control channels, is transmitted through the RUs located in the middle 302 of the system band, i.e., all RUs except for RU#1 and RU#50.
Control information transmitted in the first slot 308 of RU#1 is repeatedly transmitted through RU#50 311 in the next slot by frequency hopping, thereby obtaining frequency diversity gain. Similarly, control information transmitted using the first slot 309 of RU#50 is repeatedly transmitted through RU#1 310 in the next slot by frequency hopping. Meanwhile, several control channels are transmitted in one RU after undergoing Code Domain Multiplexing (CDM).
FIG. 4 illustrates the detailed CDM structure for control channels.
Referring to FIG. 4, ACK CHannel (ACKCH)#1 and ACKCH#2 allocated to different terminals transmit their associated ACK/NACK signals using the same Zadoff-Chu (ZC) sequence at every LB. Symbols of a ZC sequence 412 applied to ACKCH#1 are transmitted in an order of s1, s2, . . . , s12 at every LB, and symbols of a ZC sequence 414 applied to ACKCH#2 are transmitted in an order of s3, s4, . . . , s12, s1, s2. That is, the ZC sequence applied to ACKCH#2 is cyclic-shifted from the ZC sequence of ACKCH#1 by 2 symbols (Δ (Delta)=2 symbols). ZC sequences having different cyclic shift values ‘0’ 408 and Δ (Delta) 410 according to the ZC sequence characteristics having mutual orthogonality. By setting a difference between the cyclic shift values 408 and 410 to a value greater than the maximum transmission delay of a wireless transmission path, it is possible to maintain orthogonality between channels.
Corresponding ZC sequences of ACKCH#1 and ACKCH#2 are multiplied by ACK/NACK symbols b1 and b2 desired to be transmitted at every LB, respectively. Due to the orthogonality between the ZC sequences, even though ACKCH#1 and ACKCH#2 are transmitted at the same time in the same RU, a base station's receiver can detect the ACK/NACK symbols b1 and b2 of two channels without mutual interference. At LBs 405 and 406 located in the middle of a slot, Reference Signals (RSs) for channel estimation are transmitted during detection of the ACK/NACK symbols. Like the control information of ACKCH#1 and ACKCH#2, the RS is also transmitted after undergoing CDM by means of its corresponding ZC sequence. In FIG. 4, b1 and b2 are repeated over several LBs, in order to enable even the terminal located in the cell boundary to transmit an ACK/NACK signal of sufficient power to the base station.
According to a similar principle, even the COI channel transmits one modulation symbol at every LB, and different CQI channels can undergo CDM using ZC sequences having different cyclic shift values.
FIG. 5 illustrates a structure where five control channels 500-504 are multiplexed in one RU for a 0.5-ms slot.
Referring to FIG. 5, there are shown two ACK Channels, ACKCH#1 500 and ACKCH#2 501, employing coherent modulation; and three control channels of Non-Coherent Signaling Control CHannels (NCCCH)#1 502, #2 503 and #3 504 for transmitting 1-bit control information using a non-coherent modulation scheme. ACKCH#1 500 and ACKCH#2 502 transmit RS signals for channel estimation at the 2nd and 6th LBs (hereinafter, “RS LBs”) 511 and 512 (513 and 514), respectively, and transmit ACK/NACK symbols 515 at the remaining LBs (hereinafter, “control information LBs”). NCCCHs 502, 503 and 504 transmit only the control information at the 1st, 3rd, 4th, 5th, and 7th LBs.
ACKCH#1 500 and ACKCH#2 501 apply the same cyclic shift value Δ (shift of ZC) 510 to ZC sequences transmitted at each LB. Therefore, the same cyclic shift value Δ (shift of ZC) 510 is applied between the two channels 500 and 501 even at LBs 511-514 for transmission of RS signals.
For orthogonal detection of ACK/NACK symbols b1 and b2 transmitted in the two channels 500 and 501, the signals multiplexed to ZC sequences of ACKCH#1 500 and ACKCH#2 501 are multiplied by sequence symbols of N-bit orthogonal sequences Sm,n 516 (where n denotes a sequence symbol index, for n=1, . . . , N) with different indexes m in units of LBs. For instance, a Fourier sequence defined as Equation (1) can be applied as the orthogonal sequence.
                                          S                          m              ,              n                                =                      exp            ⁡                          (                              j                ⁢                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    mn                                    N                                            )                                      ,                  n          =          1                ,        …        ⁢                                  ,        N                            (        1        )            
The Fourier sequence satisfies mutual orthogonality between sequences with different indexes m, and N=5 in the structure shown in FIG. 5. Aside from the Fourier sequence, other 5-bit sequences such as ZC and Generalized Chirp-Like (GCL) sequences can also be used as the orthogonal sequence.
In the example of FIG. 5, symbols of 5-bit sequences with indexes 1 and 2 are sequentially multiplied by signals of control information LBs of ACKCH#1 and ACKCH#2, respectively. Specifically, at LB 520, each symbol of a ZC sequence applied in common to ACKCH#1 and ACKCH#2 is multiplied by an ACK/NACK symbol b1 of ACKCH#1 and the first symbol S1,1 of a Fourier sequence #1. Similarly, at LB 521, each symbol of the ZC sequence is multiplied by an ACK/NACK symbol b2 of ACKCH#2 and the first symbol S1,1 of a Fourier sequence #2.
Meanwhile, since two RS LBs 511-514 exist in one slot, 2-bit Walsh sequences with different indexes are applied to ACKCH#1 500 and ACKCH#2 501 at RS LBs 511-514. When ZC sequences with the same cyclic shift value 510 are applied to ACKCH#1 500 and ACKCH#2 501 as described above, since a length of the orthogonal sequence Sm,n is 5, three more orthogonal sequences are available. However, as stated above, since only two LBs capable of transmitting the RS exist in one slot, there is a problem in that it is not possible to generate additional RS signals other than ACKCH#1 500 and ACKCH#2 501 when applying the same ZC sequences to the control information LBs.